Blocked polyisocyanates, a process for their preparation, and their use

ABSTRACT

Partially or totally blocked polyisocyanates comprising physical mixtures of individual a) urethane-functional and/or b) biuret-functional and/or c) isocyanurate-functional aliphatic and (cyclo)aliphatic and/or cycloaliphatic diisocyanate whose isocyanate groups are blocked in such a way that there is from 0.5 to 1 mol, preferably from 0.8 to 1 mol, of 1,2,4-triazole per isocyanate equivalent.

This application is a continuation of U.S. Ser. No. 08/867,336, filedJun. 2, 1997, and issued as U.S. Pat. No. 6,051,675.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to novel partially or totally blockedpolyisocyanates with a high latent isocyanate content and highreactivity, to a process for their preparation, and to their use forproducing polyurethane (PU) polymers, especially heat-curable PU coatingsystems, and, with particular preference, low temperature-curable PUpowder coatings, and to the coatings produced accordingly.

2. Discussion of the Background

Heat-curable PU powder coatings based on partially or totally blockedpolyisocyanates and hydroxyl-containing polymers, whose softening pointis above 40° C., are widely described in the literature, such as in theDE-A documents 21 05 777, DE-25 42 191, DE- 27 35 497 (U.S. Pat. No.4,246,380), DE-28 42 641, DE-30 04 876, DE-30 39 824 and DE-31 28 743.

PU powder coatings consist essentially of a hydroxyl-containingcomponent and a polyisocyanate, whose NCO groups are partially orcompletely masked with a blocking agent, so that the OH/NCO polyadditionreaction is unable to begin at temperatures below 140° C. Only afterheating them to ≧150° C. is it possible to crosslink such PU powdercoatings, to form a coating film within a practical period of time, withremoval of the blocking agent and reaction of the OH groups with the NCOgroups.

From the large number of blocking agents, described in Houben-Weyl,Methoden der organischen Chemie [Methods of organic chemistry], VolumeXIV/2, 4th Edition, Georg Thieme Verlag, Stuttgart 1963, pages 61-70,only ε-caprolactam has become established in the industry for theintended use of the blocked polyisocyanates in the PU powder coatingsector.

To crosslink the coating, PU powder coatings based onε-caprolactam-blocked polyisocyanates require curing temperatures ofbetween 170 and 200° C.

There is therefore great interest in reducing the high curingtemperatures so as to open up powder technology fortemperature-sensitive workpieces. Likewise, of interest is the reductionof the curing times, in order to make it possible to increase theproduction rates (piece rates). Consequently, both environmental andeconomic factors are important.

Attempts have been made to achieve these aims by using oxime-blocked(cyclo)aliphatic polyisocyanates. For example, oxime-blockedpolyisocyanates and their use in PU powder coatings are described inDE-A 22 00 342, EP-A 0 432 257 and U.S. Pat. No. 3,857,818. EP-B 0 401343 describes PU powders comprising, as hardener component, an acetoneoxime-blocked trimethylolpropanetetramethylxylylene diisocyanate. EP-B 0409 745 specifies, as PU powder hardeners, 2,4-dimethyl-3-pentanoneoxime- and/or 2,6-dimethyl-4-heptanone oxime-blocked isocyanurates ofisophorone diisocyanate (IPDI), of methylene-bis-4,4'-cyclohexylisocyanate and of m- and p-tetramethylsiylylene diisocyanate.

EP-B 0 531 862 relates to a process for preparing powder coatings with aglass transition temperature of 20-80° C. by mixing A) a polyolcomponent, B) a ketoxime-blocked polyisocyanate, C) a catalystcomponent, consisting of at least one catalyst for the reaction betweenblocked NCO groups and hydroxyl groups, and optionally D) furtheradditives and auxiliaries known for powder coating technology, thepowder coatings being prepared by dissolving components A, B, C and, ifused, D homogeneously in an inert solvent or solvent mixture having aboiling point or boiling range between 50 and 150° C., and then removingthe solvent from the resulting solution.

Using oxime-blocked polyisocyanates, it is in fact possible to preparePU powder coatings whose curing temperatures are at a low level.However, a distinction must be made between transparent and pigmentedcoatings. The relatively high thermal instability of such PU powdercoatings is a disadvantage; the coatings have a tendency towardyellowing. A further disadvantage is the high level of susceptibility todefects ranging from pinholing to foaming, with the result that PUpowder coatings containing oxime-blocked polyisocyanate are ofrestricted utility and can be employed only for thin-film coating.

DE-A 28 12 252 describes 1,2,4-triazole-blocked polyisocyanates whichare employed in PU powder coatings--"they surprisingly bring aboutfurther improvement of the powder coating binders of the prior art".These are 1,2,4-triazole-blocked diisocyanates and/or polyisocyanatesthereof which carry urethane groups.

In the description of DE-A 30 33 860 , EP 0 047 452 it is stated frompage 2, line 29 to page 3, line 6 that the blockedisocyanatoisocyanurate of hexamethylene diisocyanate (HDI) is unsuitablefor use in PU powder coatings. An exception is constituted by theblocked isocyanatoisocyanurate of isophorone diisocyanate (IPDI). As EP0 047 452 goes on to show, it is possible by mixed trimerization ofthese two polyisocyanates (HDI/IPDI) to prepare products which, in theirblocked form (cf. page 8, lines 16-21), are suitable for the PU powdercoating sector, albeit with no experimental proof given. Reference ismade to the variability of the melting range as a function of theHDI/IPDI molar ratios employed; increased solvent compatibility,low-temperature flexibility, etc are mentioned--cf. page 3, lines 19-21.

DE-A 33 22 718 described blocked isocyanato isocyanurates of2-methylpentamethylene diisocyanate/2-ethylbutylene diisocyanate, andIPDI co-trimers or mixtures, HDI/IPDI co-trimers or mixtures thereofserve merely for comparison.

The object of the present invention is therefore to overcome thedisadvantages of the prior art and to provide novel partially or totallyblocked polyisocyanates which permit the preparation of both transparentand pigmented PU powder coatings which are notable for high reactivity,i.e., curing at low temperatures, and which therefore make it possibleto obtain--with film thicknesses in the range of those encountered inpractice--coatings which are free from pinholing and yellowing and areflexible despite their high network density.

SUMMARY OF THE INVENTION

The present invention therefore provides partially or totally blockedpolyisocyanates comprising physical mixtures of individual

a) urethane-functional and/or

b) biuret-functional and/or

c) isocyanurate-functional

aliphatic and (cyclo)aliphatic and/or cycloaliphatic diisocyanates whoseisocyanate groups are blocked in such a way that there is from 0.5 to 1mol, preferably from 0.8 to 1 mol, of 1,2,4-triazole per NCO equivalent.The polyisocyanates employed in accordance with the invention arephysical mixtures of individual urethanes, biurets and isocyanurates,which are employed in the proportions stated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Compounds to be regarded as polyisocyanates in the context of thisapplication are, in particular, diisocyanates of aliphatic and(cyclo)aliphatic and/or cycloaliphatic structures. Specificpolyisocyanates are described in Houben-Weyl, Methoden der organischenChemie [Methods of organic chemistry], volume 14/2 p. 61 ff. and J.Liebigs Annalen der Chemie, Volume 562, pp. 75-136, the relevantportions of which, which describe polyisocyanates, are herebyincorporated by reference polyisocyanates. In general, use is preferablymade of those diisocyanates which are readily available industrially,including aliphatic diisocyanates such as hexamethylene diisocyanate,(cycle)aliphatic diisocyanates such as isophorone diisocyanate (IPDI),and cycloaliphatic diisocyanates such as4,4'-diisocyanatodicyclohexylmethane (HMDI).

The novel physical mixtures comprises an aliphatic isocyanate componentand at least one representative from the group consisting of(cyclo)aliphatic and cycloaliphatic isocyanate components. Among(cyclo)-aliphatic diisocyanats with isocyanate groups attachedsimultaneously to aliphatic and cycloaliphatic structures, an example isIPDI. In contrast, cycloaliphatic diiscoyanates are understood as beingthose which carry only isocyanate groups attached directly to thecycloaliphatic ring. The ratio of aliphatic to (cyclo)aliphatic and/orcycloaliphatic diisocyanates may varry from 90:10 to 10:90% by weight,preferably from 75:25 to 25:75% by weight, in particular from 60:40 to40:60% by weight.

The trimers may be prepared by conventional methods known to those ofordinary skill in the art, such as those described in GB-B 1 391 066,DE-A 23 25 826, DE-26 44 684 and DE-29 16 201.

The biurets may be prepared by conventional methods known to those ofordinary skill in the art, such as those described in DE-A 23 08 015, 2437 130 and 30 07 679.

Finally, polyisocyanates in the context of the present invention arealso to be understood as being those adducts which are obtained byreacting the abovementioned monomeric, predominantly bifunctionalpolyisocyanates with the chain extenders which are common in isocyanatechemistry. Compounds of this kind are listed, for example, in DE-A 27 07770. Preference is given to polyols whose molecular weight is below 350,especially ethylene glycol (E) and trimethylolpropane (TMP). The chainextenders should be reacted with the polyisocyanates only in quantitieswhich are such that the resulting adduct has on average at least twoisocyanate groups.

The novel physical mixtures comprises in each case of an aliphaticurethane-and/or biuret- and/or isocyanurate-functional isocyanatecomponent and at least one representative from the group consisting of(cyclo)aliphatic urethane- and/or isocyanurate-functional isocyanatecomponents and/or one representative from the groups consisting ofcycloaliphatic urethane- and/or isocyanurate-functional isocyanatecomponents.

From among the large number of physical mixtures which can be preparedin accordance with the invention, the pure isocyanurate mixtures are nota subject of this invention.

The process products comprises, consequently, of urethanes and/orbiurets and/or isocyanurates together if appropriate with higheroligomers, having an NCO content of from 8 to 22% by weight, preferablyfrom 10 to 21.5% by weight, which is reduced by blocking with1,2,4-triazole to a free NCO content of ≦5% by weight, preferably ≦3% byweight, in particular ≦2% by weight.

The NCO group/blocking ratio is, in accordance with the invention, 1:0.5to 1, preferably 1:0.8 to 1.

In accordance with the invention, the latent NCO content of the1,2,4-triazole-blocked polyisocyanates is 7-l 18% by weight, preferably10-16% by weight.

The novel preparation of the partially or totally blocked physicalmixtures (polyisocyanates) can be carried out either in solvents or inbulk, and discontinuously in a reactor or continuously in a static mixeror, advantageously, in a multiscrew extruder, especially a twin-screwextruder.

Suitable solvents are only those which do not react with thepolyisocyanates, examples being ketones, such as acetone, methyl ethylketone, methyl isobutyl ketone, cyclopentanone and cyclohexanone;aromatic compounds, such as toluene, xylene, chlorobenzene andnitrobenzene; cyclic ethers, such as tetrahydrofuran; esters, such asmethyl acetate and n-butyl acetate; alphatic chlorinated hydrocarbons,such as chloroform and carbon tetrachloride; aprotic solvents, such asdimethylformamide, dimethylacetamide and dimethyl sulfoxide; andsolvents which are customarily used for solvent-containing PU coatingsystems.

The reaction of the polyisocyanate with the 1,2,4-triazole is usuallyconducted in the temperature range between 0 and 150° C. In order tocarry out blocking rapidly and completely, relatively high reactiontemperatures are preferred. On the other hand, the reaction temperaturemust be at least 10° C. below the deblocking temperature of the blockedpolyisocyanate. Preference is given to the temperature range which liesabout 15 to 25° C. below the deblocking temperature, i.e. approximatelyfrom 115 to 120° C.

In the blocking reaction it is possible for catalysts to be presentwhich favor the isocyanate polyaddition reaction, examples being tin(II)octoate, dibutyltin dilaurate (DBTL), tertiary amines etc.

The present invention additionally provides for the use of the novelcompounds for producing PU polymers, especially heat-curablesolvent-containing one-component PU coating systems, and very preferablylow temperature-curable PU powder coatings, and the coatings producedaccordingly.

The invention additionally provides for the use of the compositions aspowder coatings or as binders for powder coatings for the coating of anydesired heat-curable substrates, especially heat-sensitive workpieces,by coating methods which are known per se and are suitable for theprocessing of powder coatings. Owing to their high reactivity (lowcuring temperatures) and their excellent leveling properties, thetransparent PU powder coatings are particularly suitable as a topcoat,especially as an automotive topcoat.

Powder coatings of this kind constitute a composition comprisingpolyisocyanates blocked in accordance with the invention, ahydroxyl-containing polymer, and optionally, customary auxiliaries andadditives.

Particularly suitable hydroxyl-containing polymers are polyesters, epoxyresins and hydroxyl-containing acrylates having a molecular weight offrom 800 to 40,000.

Compounds suitable for preparing the powder coatings are preferably thepolyesters of terephthalic acid with the alcohols 1,6-hexanediol (HD),neopentyl glycol (NPG), 1,4-dimethanolcyclohexane (DMC) and2,2,2-trimethylolpropane.

Non-limiting examples of the epoxy resins which can be employed arelisted in DE-A 29 45 113, page 12, line 1 to page 13, line 26.

Non-limiting examples of suitable polyacrylate resins are described inDE-A 30 30 539, page 14, line 21 to page 15, line 26.

The mixing ratio can be varied within wide limits. The bestcoatings-related characteristics are obtained if the binder ratioconsists of 9-45% by weight crosslinking agent and 55-91% by weightpolyester, it being possible to establish an OH/NCO ratio of1:0.6-1:1.2, preferably 1:0.9-1:1.1. It is particularly advisable toemploy one equivalent of NCO of the crosslinking agent per OH equivalentof the polyhydroxy compound.

In order to raise the gelling rate of the heat-curable powder coatingsit is possible to add catalysts. Catalysts used are organotin compounds,such as dibutyltin dilaurate, Sn(II) octoate, dibutyltin maleate, etc.The amount of catalyst added is 0.1-5 parts by weight per 100 parts byweight of the hydroxyl-containing polyester.

For the preparation of PU powder coatings, the isocyanate component ismixed with the appropriate hydroxyl-containing polymer and, if desired,catalysts and also pigments and customary auxiliaries, such as fillersand leveling agents, for example silicone oil, acrylate resins, and themixture is homogenized in the melt. This can be done in appropriateequipment, such as a heatable kneading apparatus, but preferably byextrusion, aiming not to exceed an upper temperature limit of 120° C.The extruded mass is cooled to room temperature, suitably comminuted andthen ground to give the ready-to-spray powder. The application of thispowder to appropriate substrates can be carried out in accordance withknown techniques, for example electrostatic powder spraying,fluidized-bed sintering, and electrostatic fluidized-bed sintering.Following the application of the powder, the coated workpieces are curedin between 60 and 4 minutes by bringing them to a temperature of from130 to 200° C., preferably in between 25 and 5 minutes at from 140 to180° C.

Coating with the novel pulverulent coating compositions can suitably beperformed on all substrates which withstand the abovementioned curingconditions without any impairment of their mechanical properties, forexample metals, glass, ceramic, plastic or wood.

In comparison with their conventional counterparts, the PU powdercoatings obtained feature an improved behavior toward heat, UV andchemical influences; the transparent coatings exhibit excellentleveling, in particular.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXPERIMENTAL SECTION

A. Preparation of the 1,2,4-Triazole-Blocked Urethane-and/orBiuret-and/or Isocyanurate-Functional Polyisocyanates

General Preparation Procedure

The physical mixtures employed in accordance with Table 1 werehomogenized at 100-120° C. in a double-walled reactor. Prior to theaddition of the 1,2,4-triazole, the NCO content of the melt was checkedtitrimetrically and then the calculated amount of 1,2,4-triazole wasadded in portions at a rate such that the reaction temperature does notexceed 130° C. Following the addition of 1,2,4-triazole, the reactionproduct is stirred at 120° C. until the NCO content has fallen below thecalculated value, or <0.5% by weight.

If dibutyltin dilaurate (DBTL) is used, optionally, to accelerate thereaction, it is added following isocyanate homogenization and prior tothe addition of the blocking agent.

Use is made of the following industrially available products:

a) Isocyanurates, for example from

A) Huls AG NCO content 17.3±0.3% weight VESTANAT T 1890

B) Bayer AG NCO content: 21.8±0.3% by weight DESMODUR N 3300

C) The laboratory product W 1600 made from 4,4'-diisocyanatodicyclohexylmethane (HMDI) NCO content: 12.6±0.3% by weight

b) The biuret, for example from

D) Bayer AG NCO content: 21.8±0.3% by weight DESMODUR N 3200

c) The urethane-functional polyisocyanates, consisting of:

    ______________________________________                                                  Diisocyanate                                                                              OH component                                                                             NCO                                            Designation [mol] [mol] [%}                                                 ______________________________________                                        E         2 IPDI      E          16.1 ± 0.3                                  F 3 IPDI TMP 15.2 ± 0.3                                                    G 2 HMDI E 13.7 ± 0.3                                                      H 3 HMDI TMP 13.0 ± 0.3                                                  ______________________________________                                    

Preparation was carried out in accordance with known methods in PUchemistry.

                                      TABLE 1.1.2.4                               __________________________________________________________________________    Triazole-Blocked Isocyanate Mixtures                                                                 Chemical and Physical Characteristics                  Composition [% by wt.]             Glass                                      Polyioscyanates        NCO Content                                                                          Melting                                                                            Transition                                             NCO    1,2,4-                                                                            [% by weight]                                                                        Range                                                                              Temperature                                Example A                                                                           Designation                                                                         [%]                                                                              Amount                                                                            Triazole                                                                          Free                                                                             Total                                                                             [° C.]                                                                      [° C.]                              __________________________________________________________________________     1    A D --                                                                              18.2                                                                             77.0                                                                              23.0                                                                              0.5                                                                              13.9                                                                              104-107                                                                            74-90                                         2 A D -- 19.2 76.0 24.0 0.4 14.3 83-89 49-59                                  3 A D -- 19.2 77.9 22.1 1.7 14.9 77-80 47-62                                  4 A D -- 19.7 75.5 24.5 0.6 14.7 68-71 44-58                                  5 C D -- 14.2 81.1 18.9 0.4 11.3 93-97 57-69                                  6 C D -- 16.2 79.0 21.0 0.5 12.4 75-77 44-57                                  7 E B -- 17.5 77.6 22.4 0.5 13.1 94-97 58-69                                  8 E B -- 19.0 76.2 23.8 0.6 14.2 62-65 22-34                                  9 F B -- 17.4 77.7 22.3 0.5 13.2 83-86 48-60                                 10 F B -- 18.6 76.6 23.4 0.6 14.0 65-68 34-54                                 11 E D -- 17.7 77.5 22.5 0.6 13.6 81-84 45-62                                 12 E D -- 17.7 80.9 19.1 2.3 14.0 81-84 47-59                                 13 F D -- 17.6 77.5 22.5 0.6 13.3 79-82 46-61                                 14 G B -- 15.6 74.1 25.9 0.5 12.0 72-75 41-57                                 15 H B -- 15.1 80.1 19.9 0.5 11.9 77-80 37-54                                 16 H B -- 14.2 82.9 17.1 1.4 11.5 71-76 33-52                                 17 G D -- 15.0 80.3 19.7 0.4 11.7 81-83 44-59                                 18 G D -- 15.7 79.4 20.6 0.5 12.1 66-70 36-51                                 19 H D -- 15.2 79.9 20.1 0.5 11.9 72-76 40-57                                 20 A B E 18.7 76.5 23.5 0.4 14.0 85-90 48-61                                  21 A B F 19.3 75.9 24.1 0.4 14.1 88-94 52-66                                  22 A D F 18.7 76.5 23.5 0.6 13.9 80-84 45-58                                  23 A B H 17.8 77.3 22.7 0.5 13.6 86-91 51-64                                  24 A D H 17.9 77.2 22.8 0.4 13.8 90-93 52-68                                  25 B F G 17.1 78.1 21.9 0.5 13.0 73-77 42-58                                  26 D E H 17.0 78.1 21.9 0.5 13.1 70-74 39-55                                __________________________________________________________________________

Polyol Component

General Preparation Procedure

The starting components--terephthalic acid (TA), dimethyl terephthalate(DMT), 1,6-hexanediol (HD), neopentyl glycol (NPG),1,4-dimethylolcyclohexane (DMC) and trimethylolpropane (TMP)--are placedin a reactor and heated with the aid of an oil bath. After thesubstances have mostly melted, 0.5% by weight of di-n-butyltin oxide isadded as catalyst at a temperature of 160° C. Initial elimination ofmethanol takes place at a temperature of about 170° C. Over the courseof from 6 to 8 hours, the temperature is raised to 220-230° C., and thereaction is taken to completion over the course of a further 12 to 15hours. The polyester is cooled to 200° C. and is largely free fromvolatile constituents by application of a vacuum (1.33 mbar) over from30 to 45 minutes. Throughout the reaction period, the bottom product isstirred and a gentle stream of N₂ is passed through the reactionmixture.

Table 2 gives polyester compositions and commercial polyesters with thecorresponding physical and chemical characteristics.

                                      TABLE 2                                     __________________________________________________________________________                           Chemical and Physical Characteristics                  Starting Components                           Viscosity                       Example                                                                            TA DMT                                                                              HD NPG                                                                              DMC                                                                              TMP                                                                              OH Number                                                                           Acid Number                                                                          m.p. DTA  at 160° C.                 B [mol] [mol] [mol] [mol] [mol] [mol] [mgKOH/g] [mgKOH/g] [° C.]                                                   [° C.] [mPa .multidot                                                  . s]                            __________________________________________________________________________    1    10 10 6.25                                                                             10.5                                                                             2  2.9                                                                              55-60 3-4    about 75                                                                           about 50                                                                           ≈25,000                 2    CRYLCOAT ® 2392 UCB/Belgium                                                                 27-33 2-3     85-100                                                                            >50  ≈35,000                   3 URALAC ®P 1460 DSM/Netherlands 38-41   4-4.5 70-75 >50 40,000                                                        4 ALFTALAT ®AN 739                                                       Hoechst/Italy 55-60 2-4                                                       82-90 >50 24-29,000             __________________________________________________________________________

C. Polyurethane Powder Coatings

General Preparation Procedure

The comminuted products--blocked polyisocyanates (crosslinking agents),polyesters, leveling agent masterbatch and, if used, catalystmasterbatch are intimately mixed, together if appropriate with the whitepigment, in an edge runner mill and the mixture is subsequentlyhomogenized in an extruder at up to a maximum of 130° C. After it hascooled, the extrudate is crushed and ground to a particle size <100 μmusing a pinned-disk mill. The resulting powder is applied to degreased,optionally pretreated iron panels using an electrostatic powder sprayingunit at 60 kV, and the panels are baked in a drying oven at temperaturesbetween 140 and 180° C.

Leveling Agent Masterbatch

10% by weight of the leveling agent--a commercially available copolymerof butyl acrylate and 2-ethylhexyl acrylate--is homogenized in the meltin the corresponding polyester, and the melt is comminuted after it hassolidified.

Catalyst Masterbatch

5% by weight of the catalyst--DBTL--is homogenized in the melt in thecorresponding polyester, and the melt is comminuted after it hassolidified.

The abbreviations in the tables below have the following meanings:

LT=Layer thickness in μm

EI=Erichsen indentation in mm (DIN 53 156)

CH=Crosshatch test (DIN 53 151)

GG60°=Gardner gloss measurement (ASTM-D 5233)

Imp. rev.=Impact reverse in g.m

HK=Konig hardness in sec (DIN 53 157)

                                      TABLE 3                                     __________________________________________________________________________    C1 Pigmented Powder Coatings                                                  __________________________________________________________________________                  Example C1 Formulation                                                        1   2   3   4   5   6** 7   8   9   10  11  12                  __________________________________________________________________________      Crosslinking agent acc. to A 23.8 13.1 22.6 23.7 22.8 19.2 28.2 18.4                                                                  23.3 14.5 26.4                                                                20.2                  Table 1 Example () (1) (3) (3) (4) (4) (4) (5) (8) (8) (9) (10) (10)                                                                   Polyester acc.                                                               to B1 --  -- --                                                               76.3 -- -- 71.2                                                               -- -- -- 73.6                                                                 --                    Polyester acc. to B2 -- 86.9 -- -- -- -- -- -- -- 85.5 -- --                  Polyester acc. to B3 -- -- -- -- -- -- -- 81.6 -- -- -- 79.8                  Polyester acc. to B4 76.2 -- 77.4 -- 77.2 80.8 -- -- 76.7 -- -- --           Notes:                                                                        All formulations contain 40% by weight of TiO, (white pigment) and 0.5% b     weight each of leveling agent and benzoin; the OH/NCO ratio is 1:1, o)        1:0.8; x) 0.1% by weight DBTL                                            

    Coatings data                                                                   LT 64-73 67-77 71-83 67-81 72-88 69-81 65-79 74-89 67-86 69-75 64-77                                                                  67-81                 GG 60"≮ 90 90/91 89/90 89 90 90 89/90 89/90 89/91 90/91 89                                                                89/91                 CH 0 0 0 0 0 0 0 0 0 0 0 0                                                    EI >10 >10 >10 >10 >10 >10 9.7/10 >10 >10 >10 >10 >10                         Imp. rev. 460.8 345.6 576 576 691.2 345.6 460.8 806.4 >944.6 691.2 576                                                                806.4               __________________________________________________________________________     Notes                                                                         Curing conditions: 180° C./6-8', 170° C./10-12', 160.degree     C./15-25', 150° C./25-30                                          

    Example C1 Formulation                                                                      13  14***                                                                             15  16  17***                                                                             18  19  20  21  22  23  24***               __________________________________________________________________________      Crosslinking agent acc. to A 24.2 20.3 25.6 17.1 14.1 26.7 21.9 26.4                                                                  26.7 14.2 23.9                                                                20.1                  Table 1 Example () (11) (11) (13) (14) (14) (15) (16) (18) (20) (23)                                                                  (24) (24)                                                                      Polyester acc.                                                               to B1 --  --                                                                  74.4 -- -- --                                                                 -- -- -- -- --                                                                --                    Polyester acc. to B2 -- -- -- 82.9 85.9 -- -- -- -- 85.8 -- --                Polyester acc. to B3 -- -- -- -- -- -- 78.1 -- -- -- -- --                    Polyester acc. to B4 75.8 79.7 -- -- -- 73.3 -- 73.6 73.3 -- 76.1 79.9       Notes:                                                                        All formulations contain 40% by weight of TiO, (white pigment) and 0.5% b     weight each of leveling agent and benzoin; the OH/NCO ratio is 1:1, o)        1:0.8; x) 0.1% by weight DBTL                                            

    Coatings data                                                                   LT 62-66 57-68 67-81 67-81 64-79 52-67 63-78 71-84 74-89 59-73 64-88                                                                  61-83                 GG 60"≮ 90/91 90 88/89 90 90 91 89/90 89/90 90 90 90 90/91                                                                 CH 0 0 0 0 0 0                                                               0 0 0 0 0 0                                                                    EI >10 >10 >10                                                               >10 >10 >10 >10                                                               9.6/9.9 >10                                                                   9.7/9.9 >10 >10       Imp. rev. >944.6 576 345.6 806.4 460.8 >944.6 460.8 460.8 576 345.6 576                                                               691.2               __________________________________________________________________________     Notes                                                                         Curing conditions: 180° C./6-8', 170° C./10-12', 160.degree     C./15-25', 150° C./25-30                                          

    __________________________________________________________________________    C2 Transparent Powder Coating                                                   Following the procedure described,                                            the transparent powder coatings too were prepared, applied and baked at     between 180 and 140° C.                                                             Example C2 Formulation                                                        1   2   3** 4   5   6   7   8** 9                                __________________________________________________________________________    Crosslinking agent acc. to A                                                               13.9                                                                              22.8                                                                              22.8                                                                              13.7                                                                              23.3                                                                              23.7                                                                              15.9                                                                              15.9                                                                              23.9                               Table 1 () (1) (4) (4) (8) (8) (10) (15) (15) (24)                            Polyester acc. to B2 86.1 --  -- 86.3 -- -- 84.1 84.1 --                      Polyester acc. to B4 -- 77.2 77.2 -- 76.7 76.3 -- -- 76.1                    Notes:                                                                        All formulations contain 0.5% by weight each of leveling agent and 0.3% b     weight of benzoin; the OH/NCO ratio is 1:1, o) 1:0.8; x) 0.1% by weight       DBTL                                                                     

    Coating data                                                                    LT 45-60 60-73 53-67 57-71 66-78 63-73 58-70 61-74 59-71                      HK 191 194 197 198 214 195 197 199 207                                        CH 0 0 0 0 0 0 0 0 0                                                          EI >10 >10 >10 >10 >10 >10 >10 >10 >10                                      __________________________________________________________________________     Notes                                                                         Curing conditions: 180° C./5', 170° C./6-9', 160°        C./10-15', 150° C./15-25', 140° C./25-30';                 

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

This application is based on German patent application 196 26 886.9filed in the German Patent Office on Jul. 4, 1996 the entire contents ofwhich are hereby incorporated by reference.

What is claimed is:
 1. A polyurethane powder coating comprising apartially or totally 1,2,4-triazole-blocked polyisocyanate and ahydroxyl-containing polyester, wherein the blocked polyisocyanate is aphysical mixture of (1) an aliphatic urethane- orisocyanurate-functional polyisocyanate and (2) a (cyclo)aliphatic orcycloaliphatic urethane-or isocyanurate-functional polyisocyanate,wherein the physical mixture of the polyisocyanates contains no mixtureof two or more isocyanurate-functional polyisocyanates, wherein theisocyanate groups are blocked in such a way that there is from 0.5 to 1mol of 1,2,4-triazol per isocyanate equivalent, and the ratio by weightof aliphatic to (cyclo)aliphatic and cycloaliphatic polyisocyanates isfrom 90 to 10 to 10 to 90, and the hydroxyl-containing polyester has afunctionality of ≧2, an OH number of from 30 to 150 mg KOH/g, aviscosity <40,000 mPa.s at 160° C. and a melting point of from 75 to100° C., and wherein either(A) component (1) is an aliphaticisocyanurate-functional polyisocyanate, or (B) component (2) is acycloaliphatic urethane-functional polyisocyanate, or (C) component (2)is a cycloaliphatic isocyanurate-functional polyisocyanate.
 2. Thepolyurethane powder coating of claim 1, wherein component (1) is analiphatic urethane-functional polyisocyanate, and wherein(A) component(2) is a cycloaliphatic urethane-functional polyisocyanate, or (B)component (2) is a cycloaliphatic isocyanurate-functionalpolyisocyanate.
 3. The polyurethane powder coating of claim 1, whereincomponent (1) is an aliphatic isocyanurate-functional polyisocyanate. 4.The polyurethane powder coating of claim 1, wherein component (2) is acycloaliphatic urethane-functional polyisocyanate.
 5. The polyurethanepowder coating of claim 1, wherein component (2) is a cycloaliphaticisocyanurate-functional polyisocyanate.
 6. The polyurethane powdercoating of claim 2, wherein component (2) is a cycloaliphaticurethane-functional polyisocyanate.
 7. The polyurethane powder coatingof claim 2, wherein component (2) is a cycloaliphaticisocyanurate-functional polyisocyanate.
 8. The polyurethane powdercoating of claim 3, wherein component (2) is (cyclo)aliphaticpolyisocyanate.
 9. The polyurethane powder coating of claim 3, whereincomponent (2) is a cycloaliphatic urethane-functional polyisocyanate.